This invention relates to an improved horizontal mirror motor for driving a rotary multi-mirror in an electrophotographic printer.
FIG. 1 is an explanatory diagram outlining the arrangement of an electrophotographic printer. A photosensitive drum 8 has characteristics such that has a high dielectric at dark regions, but its charges dissipate upon the reception of light. Thus, the drum 8 is similar to a photographic film. The surface of the drum 8 is exposed to the corona of a charger 7, and is then irradiated by a laser beam in correspondence to printing data. As a result, the charges at that part of the surface to which the laser beam has been applied dissipate, so that the latent image of the printing data is formed electrostatically.
The above-described process of applying the laser beam will now be described in more detail. The continuous laser beam from a laser oscillator 1 is divided into dots according to the printing data by a modulator 2, and is then applied through a rotary multi-mirror 3 and a reflecting mirror 6 to the surface of the photo-sensitive drum 8 in such a manner as to scan the latter, as a result of which the latent image of the printing data is formed thereon.
The latent image is developed with printing ink, or toner, by a developing unit 9. In succession, a transferring unit 11 applies a high voltage to the rear surface of a printing sheet conveyed by a traction drive 10, so that toner is transferred thereto. The toner is thermally fixed to the printing sheet 13 by a fixing unit (not shown). In the above-described electrophotographic printing method, characters and ruled lines on forms can be printed simultaneously, and therefore it is unnecessary to prepare many kinds of printed forms in advance, which contributes greatly to economical use of paper sheets.
However, the conventional printer of this type suffers from a problem that, as shown in the right hand portion of FIG. 2, a vertical ruled line of the form has a poor appearance, waving by about .DELTA.x=0.1 to 0.2 mm. The reason for this is that the number of revolutions per minute of the mirror motor driving the rotary multi-mirror 3 changes slightly by 0.02 to 0.04%.
FIG. 3 is a diagram showing a process of forming a latent image, in which a laser beam 5 is scanned over a photo-sensitive drum 8 by a rotary multi-mirror 3. In this process, a beam detecting sensor 15 is employed for determining the start point of each scanning line. When the number of revolutions per minute of the mirror motor changes, the scanning distance LP which is scanned to print a desired ruled line is changed, as a result of which the vertical ruled line appears wavy.
FIG. 4 shows one example of a horizontal mirror motor. The motor employs dynamic pressure pneumatic bearings in order to reduce bearing torque as much as possible to decrease any variation in the number of revolutions per minute, and in order to eliminate troublesome replacement and maintenance of bearings.
As shown in FIG. 4, a rotary multi-mirror 3 is fixedly mounted on a mirror shaft 22. The mirror shaft 22 is supported in the radial direction by dynamic pressure pneumatic bearings each made up of three tilting pads 21 and in the thrust direction by repulsive magnets 25a and 25b. A rotor 23 and a stator 24 form the motor.
FIG. 5 is an enlarged view of the dynamic pressure type bearing section of FIG. 4. In FIG. 5, the pneumatic bearing of a pad 20c provides a floating force F.sub.1 which corresponds to a moment (WP.times.1.sub.1W) provided by the weight (WP) of the pad 20c. (F.sub.1 .times.1.sub.1F =WP.times.1.sub.1W).
The relation between the air gap g of the pneumatic bearing and the floating force F is as shown in FIG. 6. A considerably large floating force F.sub.1 is provided by an air gap g.sub.1 in FIG. 6, which gap is very small. When the air gap is eliminated by an external disturbance, the number of revolutions per minute is changed by the frictional resistance between the pad and the shaft.